1. Field of the Invention
The present invention relates to a dispersion compensating optical fiber for compensating for wavelength dispersion and a dispersion slope of an optical transmission path in an optical communication system, and particularly to a dispersion compensating optical fiber for achieving an increased slope rate.
2. Related Art
In an optical communication system, there is a case where an optical signal of 1.55 xcexcm wavelength band is transmitted over a long distance, through an optical transmission path such as constituted of a single mode fiber (SMF) which exhibits zero-dispersion near 1.3 xcexcm wavelength. In such a case, the optical transmission path has wavelength dispersion on the order of about 16 ps/nm/km for the optical signal of 1.55 xcexcm band, resulting in a problem that the transmission of optical signal over a long distance at a high speed leads to accumulation of wavelength dispersion to thereby distort the waveform of the optical signal. For example, if an optical signal is transmitted over a distance of 100 km, a positive wavelength dispersion of about 1,600 ps/nm is accumulated, thereby requiring compensation therefor. Further, when a wavelength division multiplexing (WDM) transmission is performed using the aforementioned optical transmission path, it is also required to compensate for a dispersion slope of the optical transmission path. Concretely, it is known that a 1.3 xcexcm zero-dispersion SMF has a positive dispersion slope on the order of about 0.05 ps/nm2/km.
Conventionally, to compensate for the wavelength dispersion of an optical transmission path having such a positive wavelength dispersion and a positive dispersion slope, there has been utilized a dispersion compensating optical fiber (DCF) having a negative wavelength dispersion and a negative dispersion slope. As conventional dispersion compensating optical fibers, various structures have been concretely proposed.
For example, there has been disclosed a basic structure having a xe2x80x9cWxe2x80x9d shaped refractive index profile as shown in FIG. 8 in the conventional dispersion compensating optical fibers such as described in Japanese Unexamined Patent Publication Nos. 7-261048, 8-136758, 8-313750, 10253847, 9-318833 and 11-95056. Concretely, the conventional dispersion compensating optical fiber has the basic structure wherein a low refractive index portion 52 is formed on the periphery of a core portion 51 positioned at the center of the optical fiber, and a cladding portion 53 is formed on the periphery of the low refractive index portion 52. The core portion 51 has a high refractive index of which specific refractive index difference to pure silica is a positive value, and the low refractive index portion 52 has a low refractive index of which specific refractive index difference refractive index is a negative value to pure silica, while the cladding portion 53 is formed of pure silica (i.e., its specific refractive index difference is zero). By forming the basic structure in such a xe2x80x9cWxe2x80x9d shaped refractive index profile, a dispersion compensating optical fiber having a negative wavelength dispersion and a negative dispersion slope is realized.
Further, as shown in FIG. 9, there has been also disclosed a structure wherein, in the cladding portion 53, a part adjacent to the low refractive index portion 52 has a slightly high refractive index in the xe2x80x9cWxe2x80x9d shaped refractive index profile, in conventional dispersion compensating optical fibers such as described in Japanese Unexamined Patent Publication Nos. 2000-47048, 6-222235, 7-270636, 11-507445 and 10300965. By forming such a structure, there is realized a dispersion compensating optical fiber capable of such as reducing transmission loss.
Meantime, in many cases, there is adopted a dispersion shifted fiber (DSF) as an optical transmission path in an optical communication system for transmitting an optical signal of 1.55 xcexcm band over a long distance. This DSF is an optical fiber obtained by shifting the zero-dispersion wavelength of the 1.3 xcexcm zero-dispersion SMF to the 1.55 xcexcm band and being specialized in optical transmission in 1.55 xcexcm band where the transmission loss of silica optical fiber becomes a minimum. However, when performing a WDM transmission in 1.55 xcexcm band through an optical transmission path adopting a DSF, the smaller wavelength dispersion in 1.55 xcexcm band rather leads to a defective susceptibility to a nonlinear optical effect. As such, there has been proposed a system utilizing a non-zero dispersion shifted fiber (NZ-DSF) wherein the zero-dispersion wavelength of DSF has been intentionally shifted to the outside of a signal light wavelength band. Since such an NZ-DSF has a finite wavelength dispersion and a finite dispersion slope in the signal light wavelength band of 1.55 xcexcm, it is required to compensate for the wavelength dispersion and dispersion slope in a long distance high-speed transmission of a WDM optical signal.
To compensate corresponding to the wavelength dispersion and dispersion slope of the NZ-DSF, there is required a dispersion compensating optical fiber having a large xe2x80x9cslope ratexe2x80x9d defined by a value obtained by dividing the dispersion slope by the value of the wavelength dispersion compared to the 1.3 xcexcm zero-dispersion SMF. However, it is not easy to achieve an increase in slope rate in the dispersion compensating optical fiber having the aforementioned conventional structure. As one concrete way to increase the slope rate, it is considered to reduce the refractive index of the low refractive index portion 52. According to the technique to date, however, it is difficult to reduce the refractive index down to a desired level, due to such as a manufactural problem. Even when a dispersion compensating optical fiber having a greater slope rate corresponding to the NZ-DSF can realized, such a dispersion compensating optical fiber should be so expensive.
Further, in the conventional dispersion compensating optical fiber having the structure as shown in FIG. 9, there are caused light in higher mode propagated through the part with higher refractive index of the cladding portion 53. If this light in higher mode is sent out to the transmission path, then there is caused a problem in that the signal waveform of the transmitted light would be deteriorated.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a dispersion compensating optical fiber having a negative wavelength dispersion and a negative dispersion slope and exhibiting a large slope rate. It is a further object of the present invention to provide a dispersion compensating optical fiber for eliminating higher mode optical components to thereby transmit fundamental mode optical signals only.
To achieve the aforementioned object, one aspect of the dispersion compensating optical fiber according to the present invention includes: a core portion positioned at the center of the optical fiber; a low refractive index portion provided at the periphery of the core portion and having a refractive index lower than that of the core portion; and an intermediate refractive index portion provided at the periphery of the low refractive index portion and having a refractive index higher than that of the low refractive index portion and lower than that of the core portion, and has a negative wavelength dispersion and a negative dispersion slope, wherein the intermediate refractive index portion is formed by doping to pure silica a dopant for increasing a refractive index such that a first specific refractive index difference (xcex943) thereof relative to pure silica has a positive value, the low refractive index portion is formed by doping to pure silica a dopant for reducing a refractive index such that a value obtained by subtracting the first specific refractive index difference (xcex943) of the intermediate refractive index portion from a second specific refractive index difference (xcex942) relative to pure silica becomes xe2x88x920.8% or less, the core portion is formed by doping to pure silica a dopant for increasing a refractive index such that a value obtained by subtracting the first specific refractive index difference (xcex943) of the intermediate refractive index portion from a third specific refractive index difference (xcex941) relative to pure silica becomes 1.5% or more, and a slope rate obtained by dividing the dispersion slope by a value of the wavelength dispersion becomes 0.01 or more.
According to the dispersion compensating optical fiber having such a constitution, the refractive index of the intermediate refractive index portion is raised so as to reduce the nominal specific refractive index of the low refractive index portion, to thereby obtain an effect identically with the situation where the refractive index of the low refractive index portion is reduced. Further, the value (xcex942-xcex943) obtained by subtracting the first specific refractive index difference (xcex943) of the intermediate refractive index portion from the specific second specific refractive index difference (xcex942) of the low refractive index portion is rendered to be xe2x88x920.8% or less, and the value (xcex941-xcex943) obtained by subtracting the first specific refractive index difference (xcex943) of the intermediate refractive index portion from the third specific refractive index difference (xcex941) of the core portion is rendered to be 1.5% or more, to thereby allow the slope rate to be 0.01 or more without further reducing the refractive index of the low refractive index portion. In this way, it becomes possible to readily realize a dispersion compensating optical fiber suitable for compensating for a wavelength dispersion and a dispersion slope of such as an NZ-DSF, at a reduced cost.
Further, another aspect of the dispersion compensating optical fiber according to the present invention includes: a core portion positioned at the center of the optical fiber; a low refractive index portion provided at the periphery of the core portion and having a refractive index lower than that of the core portion; and an intermediate refractive index portion provided at the periphery of the low refractive index portion and having a refractive index higher than that of the low refractive index portion and lower than that of the core portion, and has a negative wavelength dispersion and a negative dispersion slope, wherein the dispersion compensating optical fiber further comprises a higher mode light eliminating portion for eliminating higher mode optical components propagated through the intermediate refractive index portion.
According to such a constitution, higher mode optical components propagated through the intermediate refractive index portion are eliminated by the higher mode light eliminating portion. In this way, only the fundamental mode optical signal can be sent out to the transmission path connected to the dispersion compensating optical fiber, to thereby allow the reduction of waveform deterioration in transmitted light.
Further, in the aforementioned dispersion compensating optical fiber having the higher mode light eliminating portion, the constitution may be such that the intermediate refractive index portion is formed by doping to pure silica a dopant for increasing a refractive index such that a first specific refractive index difference (xcex943) thereof relative to pure silica has a positive value, the low refractive index portion is formed by doping to pure silica a dopant for reducing a refractive index such that a value obtained by subtracting the first specific refractive index difference (xcex943) of the intermediate refractive index portion from a second specific refractive index difference (xcex942) relative to pure silica becomes xe2x88x920.8% or less, the core portion is formed by doping to pure silica a dopant for increasing a refractive index such that a value obtained by subtracting the first specific refractive index difference (xcex943) of the intermediate refractive index portion from a third specific refractive index difference (xcex941) relative to pure silica becomes 1.5% or more, and a slope rate obtained by dividing the dispersion slope by a value of the wavelength dispersion value becomes 0.01 or more.
According to the dispersion compensating optical fiber having such a constitution, the refractive index of the intermediate refractive index portion is raised so as to reduce the nominal specific refractive index of the low refractive index portion, the value (xcex942-xcex943) is rendered to be xe2x88x920.8% or less, and the value (xcex941-xcex943) is rendered to be 1.5% or more, to thereby allow the slope rate to be 0.01 or more without further reducing the refractive index of the low refractive index portion. In this way, it becomes possible to readily realize a dispersion compensating optical fiber suitable for compensating for a wavelength dispersion and a dispersion slope of such as an NZ-DSF, at a reduced cost.
Moreover, in the aforementioned dispersion compensating optical fiber, the higher mode light eliminating portion may be formed by continuously forming the intermediate refractive index portion up to the outermost layer of the optical fiber, to thereby eliminate higher mode optical components.
In this way, by continuously forming the intermediate refractive index portion having a raised refractive index up to the outermost layer of the fiber, those higher mode optical components propagated through the intermediate refractive index portion become extremely susceptible to a bend loss so that the higher mode optical components are eliminated by leaking to the outside of the dispersion compensating optical fiber.
Further, when the aforementioned dispersion compensating optical fiber includes a cladding portion, which is provided at the periphery of the intermediate refractive index portion and has a refractive index lower than that of the intermediate refractive index portion, the higher mode light eliminating portion may be formed by connecting, to a predetermined position, an optical path having a mode field diameter smaller than that of an optical transmission path to which the dispersion compensating optical fiber is connected, so as to eliminate higher mode optical components. Moreover, the optical path preferably has a mode field diameter smaller than an inner diameter of the intermediate refractive index portion.
According to such a constitution, when an optical signal sent from a preceding optical transmission path is input into the dispersion compensating optical fiber, higher mode optical components are propagated through the intermediate refractive index portion. However, these higher mode optical components are sent to an area outside the mode field of a succeeding optical transmission path, by passing through the optical path of the higher mode light eliminating portion. In this way, the higher mode optical components are eliminated from the optical signal passed through the dispersion compensating optical fiber, so that only the fundamental mode optical signal is transmitted to the succeeding optical transmission path.
Further, when the dispersion compensating optical fiber includes a cladding portion, which is provided at the periphery of the intermediate refractive index portion and has a refractive index lower than that of the intermediate refractive index portion, the higher mode light eliminating portion may be formed by setting a refractive index of the periphery portion of the intermediate refractive index portion to be higher than that of the remaining portion of the intermediate refractive index portion, to thereby eliminate higher mode optical components.
According to such a constitution, higher mode optical components propagated through the intermediate refractive index portion tend to travel the periphery portion having the higher refractive index. Since the periphery portion of the intermediate refractive index portion is at a position separated from the core portion, higher mode optical components are sent to an area outside the mode field of a succeeding optical transmission path. In this way, the higher mode optical components are eliminated from the optical signal passed through the dispersion compensating optical fiber, so that only the fundamental mode optical signal is transmitted to the succeeding optical transmission path.
Alternatively, when the dispersion compensating optical fiber includes a cladding portion, which is provided at the periphery of the intermediate refractive index portion and has a refractive index lower than that of the intermediate refractive index portion, the higher mode light eliminating portion may be formed by providing the intermediate refractive index portion with an absorber for absorbing light of 1.55 xcexcm band, to thereby eliminate higher mode optical components. Concretely, the absorber may be arranged in an area sufficiently separated from the mode field of the fundamental mode.